1. Field of the Invention
The present invention relates to a fail-safe automatic sliding operation control apparatus for a press in which the safety of the operator is ensured during sliding operation of the press, and the automatic operation of the slide is made possible.
2. Description of the Related Art
In a press control system, a serious accident may result, should a malfunction occur in the control system. Therefore, it is desired that such a control circuit, which is related to a sliding operation of a press, have a fail-safe constitution. In the following description, the term "fail-safe constitution" refers to those system constitutions for sensors or for control circuits which will never indicate "safety" under a dangerous situation; and those constitutions, in the case of electric/electronic circuits, in which output signals are never generated when a fault has occurred in the circuits.
As shown in FIG. 1, the conventional press has a slide 2 which is reciprocally moved in the vertical direction by means of a crank mechanism (not shown) within a body 1. The slide 2 is located above a bolster 3, and an operation button 4 is provided at the front side of the body 1. Provided between the bolster 3 and the operation button 4 is an optical beam type safety apparatus 5 (referred to as an "optical curtain" hereinafter) for confirming a safe status in the dangerous zone above the bolster 3. Reference numerals 6 and 7 designate metal molds (upper and lower types) which are set on the slide 2 side and bolster 3 side, respectively.
In the operational control for a safety one cycle in such a conventional press, the safe status of the operator is confirmed by means of the optical curtain 5. The slide 2 is lowered only when the operation button 4 is kept pressed by the operator, and the slide 2 is automatically moved upwardly during its rising interval. The operation button 4 is manipulated at each reciprocating motion of the slide 2. Typically, a brake is actuated at a predetermined position in the slide rising interval for each reciprocating motion of the slide 2 so as to confirm the brake function, i.e., to confirm that the slide can be stopped in the vicinity of a top dead center (within a predetermined crank angle range) (overrun monitoring function).
Shown in FIG. 2 is a positional relationship between the optical curtain 5 and the operation button 4 of the press shown in FIG. 1.
In a normal working state, the zone D within a one-dot chain line is a danger zone (as defined according to EN292-1 or ISO/TR12100-1) which is defined by the optical curtain 5 at the front of body 1 and by both sides of the bolster 3. The bolster is typically enclosed by means of block fences for preventing operators from entering the press. This danger zone D is a region determined on the basis of the speed of the slide and a moving speed of an operator, and represents the minimum region required for ensuring the safe status of operators. This region, represented by a distance between the bolster 3 and the optical curtain 5 when viewed from the front of the press, must remain clear, which means that the slide will be suddenly stopped if a hand of the operator enters within this region during the lowering of the slide. Further, the distance L between the optical curtain 5 and the operation button 4 represents a distance by which the hand of the operator should be spaced or drawn from the danger zone D as a re-confirmation of safe status just before the actuation of the slide after the safe status (absence of hand) in the danger zone D has been signaled. The reference character D.sub.N designates a further danger zone which includes the distance L.
The "safety one cycle operation of the press" means that the operator's hands should normally remain outside the danger zone D.sub.N, i.e., the hands should remain on the operation button (or outside the operation button) during the lowering motion of the slide. Thus, in the safety one cycle operation, the optical curtain 5 detects a slide sudden stop condition, while the operation button 4 provides a slide actuation condition.
There will be described hereinafter the features in the safety cycle operation utilizing the operation button.
1) Utilization of Operation Button
The operation button is utilized only for the lowering of the slide, the rising of the slide being automatically effected. In other words, the lowering motion period of the slide is a working stroke involving danger (dangerous stroke), and the rising motion period of the slide does not involve danger, so that the operation button is utilized for the dangerous stroke only.
2) Position of Operation Button
The position of the operation button acts to ensure the safe status of the operator outside the danger zone on the bolster over the optical curtain, and that the hands of the operator are spaced from the optical curtain by a predetermined distance. Thus, lowering of the slide is permitted under this situation only. In addition, the reason why the operation button is to be operated by both hands is to indicate that both hands of the operator are located at this distance.
3) Existence of Operation Button
The operation button is provided for signaling and implementing the operator's intention to actuate the slide, and acts as a press side sensor for confirming this intention.
4) Structure of Operation Button
In an ON/OFF operation for electric current I making use of an operation button, there are two possible implementations. One is to allow the current I to flow by turning ON the contact points when the operation button is pushed as shown in FIG. 3, and the other is to cut off the current I by turning OFF the contact points when the operation button is pushed as shown in FIG. 4. In the structure of FIG. 3, should the contact points not make contact with each other when the operation button is pushed, the current I does not flow. Thus, in the case of operating a slide by means of the current I, the slide will not move downwardly even if the operation button is pushed, so that the slide operates with a safety mechanism. In the structure of FIG. 4, if a disconnection in the wire for the current I should occur, current will continue to flow as if the operation button were pushed, resulting in lowering motion of the slide. In view of the above, there is adopted an operation button having the structure shown in FIG. 3, for operation of a fail-safe press.
The operation button in FIG. 3 is constructed to be pushed by the operator's hand such that the contact points are contacted with each other by this force, thereby allowing flow of the current I. Namely, the logical structure is such that the operator's energy for pushing the operation button is converted into the energy of current I for driving the slide.
Shown in FIGS. 5 and 6 are examples of sensors having logical structures identical with those in FIGS. 3 and 4, respectively.
FIG. 5 shows a logical structure identical with that of the operation button shown in FIG. 3, i.e., an example of a fail-safe sensor.
In FIG. 5, a sensor 10 is provided with a detection part 12 which has a reflected light detection type sensor substantially at the center of a cover 11. When the operator's hand 13 is positioned above the detection part 12 (corresponding to the pushing of the operation button), the light beam, which is normally emitted from the detection part 12, is reflected by the operator's hand 13 and received, whereby the existence of the operator's hand 13 is detected (this detection triggers the generation of current I). Thus, in the sensor 10 of FIG. 5, the generation of reflected light corresponds to the force of an operator's hand when pushing the operation button; signal energy from light reception is generated when the operator's hand 13 is positioned above the detection part 12, and the current I is generated by this energy.
FIG. 6 shows an example of a sensor corresponding to the operation button of FIG. 4.
In FIG. 6, a sensor 10' is provided with a light emitting part 14 and a light receiving part 15, which are disposed on opposing sides of the cover 11'. When a light beam Be is blocked or intercepted by the operator's hand 13, a light detection output signal of the light receiving part 15 disappears which, in turn, shuts down the output current, i.e., current I in FIG. 4. In this sensor 10', failure of either of the light emitting or receiving elements results in disappearance of the current I. However, the sensor structure in FIG. 6 does not constitute a fail-safe system because a possibly dangerous output signal may be erroneously generated under a failed condition.
Summarizing the aforementioned functions of the operation button in a safety one cycle operation, the following Table 1 is obtained.
TABLE 1 ______________________________________ Function ______________________________________ Utilization of operation To be used for action button (operation by both involving danger (only for hands) lowering of slide) Information attributed to the operation button: Position of Button; Confirmation of distance of hands from optical curtain; Existence of Button; and Notification of operator's intention to actuate the slide; Structure of Button Conversion of the existence of hands into energy to thereby generate another energy for permitting the lowering motion operation of slide. ______________________________________
Meanwhile, a mechanism called "interlock" permits an actuation of the relevant machine only when the zone regarded as dangerous is safe. However, "interlock" as defined by the European Standard EN 1088 or EN 292-1 does not necessarily permit actuation of the slide even when the danger zone D of FIG. 2 is safe. Namely, it permits the actuation of the slide only when the safe status (absence of hands from danger zone D) is positively re-confirmed. This reconfirmation is accomplished through use of the operation button in the safety one cycle operation. The safe status to be re-confirmed by this operation button corresponds to that information attributed to the operation button shown in Table 1.
In the safety one cycle operation of the conventional press, should the operator's hand have been placed onto the bolster during the lowering of the slide, the operation button will, of course, be released or turned OFF, initiating the function of the normal brake. Thereafter, the optical curtain is intercepted, initiating the function of the emergency brake. Thus, the slide stop operation is doubly initiated, first by the operator's rejection of operation and second by the emergency brake responsive to breach of the optical curtain.
However, the operator may suffer physical ailments such as tenosynonitis when the frequency of pushing the operation button is increased. Thus, there is demanded an operation system which does not impose burden onto the operator.
If the sensor 10 in FIG. 5 is adopted, the action of pushing an operation button can be omitted. However, such an action is still required to locate the hand 13 on the detection part 12 of sensor 10 at each reciprocating motion of the slide, so that the working state itself is not greatly improved. Rather, the working state may be even worse due to the action of the hand necessary to fit within the concave portion.
Further, there may be envisaged an automatic operation control for the slide, which depends on the optical curtain 5 of FIG. 1 only. However, safety problems are then involved, as described hereinafter.
The actuation of the slide in the absence of a both-hand type operation button means that all of the functions in the operation button indicated in Table 1 are not provided. As such, the slide 2 is immediately lowered based only on the absence of the operator's hand from the danger zone D, i.e., the slide is lowered as soon as the operator's hands are withdrawn from the danger zone D after manually inserting a work between the upper and lower metal molds 6 and 7.
This method involves the following problems, as compared to the aforementioned conventional safety one cycle operation.
(1) The operator is spaced from the danger zone D, but is still standing very closely, so that the operator may erroneously or inadvertently enter the danger zone D. Namely, the region in which the operator must exist is not limited.
(2) The operator is not bound or restricted during the lowering of the slide. Thus, the operator's behavior can not be predicted.
(3) The operator is unable to signal an intention to actuate the slide. Due to the lack of communication between the operator and the press, the lowering of the slide may occur contrary to the operator's intention.
(4) The device for slide stoppage is embodied in a single element, assuming there is only one operator.
(5) The timing as to when the slide of the press shall be lowered is not indicated.
Thus, it is necessary to provide the function of the conventional operation button by some procedure. As one such procedure, there is envisaged a procedure to monitor the position of the operator's hand such as by means of a CCD camera. However, this requires complicated information processing capable of consistently judging the movement of the hand, without failure. Further, it becomes expensive.
Then, there has been conventionally proposed a light-beam type safety apparatus for automating the operation of the press, as shown in Japanese Examined Patent Publication No. 7-92193. In this apparatus, there are provided a plurality of layers of optical curtains from the outside toward the inside of the press, and the entrance and escape motions of the human body into and from the danger zone are detected by the interceptive configuration of the plurality of layers of optical curtains. However, this light-beam type safety apparatus is not constituted in a fail-safe manner.
The reasons thereof will be explained hereinafter, in detail.
Firstly, there will be described a constitutional principle of a fail-safe optical curtain.
Shown in FIGS. 7 through 9 are constitutional examples of optical curtains. It is assumed that the light receiver does not have a function of NOT operation, and the number of light beams is one.
In the optical curtain of FIG. 7, when a person enters the danger zone (danger zone D of FIG. 2), the light beam is intercepted so that the light receiver does not receive it.
In the optical curtain of FIG. 8, when a person enters the danger zone, the light beam reflected by the person is received by the light receiver. Thus, when the presence/absence of a person and those of input/output signals of the light receiver are represented by binary logical values, "1" and "0", respectively, Tables 2 and 3 are obtained for the optical curtains of FIG. 7 and FIG.8, respectively. In these tables, the logical value is given in the parentheses.
TABLE 2 ______________________________________ Presence/Absence of Input of Light Output of Light Person Receiver Receiver ______________________________________ Absence (1) Yes (1) Yes (1) Presence (0) No (0) No (0) ______________________________________
TABLE 3 ______________________________________ Presence/ Absence of Input of Light Output of Output of NOT Person Receiver Light Receiver circuit ______________________________________ Absence (1) No (0) No (0) Yes (1) Presence (0) Yes (1) Yes (1) No (0) ______________________________________
If safety, the absence of a person from the danger zone, is to be represented by a logical value "1", the constitution of the optical curtain of FIG. 8 requires a NOT circuit N which performs a NOT operation for the output of the light receiver, as depicted by a dotted line.
The constitution of the optical curtain of FIG. 8 is difficult to adopt as a fail-safe one, since it involves the following problems.
(1) In case of failure of the light emitter (such as light emitting element), if the circuit of the light receiver is operating normally, the logical value "1" is not generated from the light receiver irrespectively of the presence of a person in the danger zone. At this time, the output signal of NOT circuit N has a logical value "1" which represents safe status (the absence of a person in the danger zone), failing to indicate the possible danger (logical value "0").
(2) Even if the light emitter and light receiver are operating normally, when a disconnection fault has occurred in the input line of NOT circuit N, the output of NOT circuit N continually has a logical value "1" to represent safe status, irrespectively of the state in the danger zone.
For at least these reasons, if a fail-safe system is to be provided, there should not be adopted such a constitution as shown in FIG. 8 where safe status is represented by either of binary values derived from a NOT circuit.
Meanwhile, in the structure of FIG. 7, the input/output relation of the light receiver, the output of which is used to represent safe status (logical value "1"), is realized in a logically monotonous (or united) manner. Namely, the logical relationship, i.e., the logical value, of the output is the same as that of the input. This type of logical structure is the basic principle of signal processing in a fail-safe system. This basic principle is known from U.S. Pat. No. 5,345,138.
In practice, the output signal from the light receiver may be amplified, as shown in FIG. 9. In this case, the amplifier should also have a fail-safe characteristic (where an output signal is not generated upon failure). A fail-safe amplifier is known from International Unexamined Patent Publication WO 94/23303. Further, known from International Unexamined Patent Publications WO 93/23772 and WO/95/10789 is a fail-safe optical sensor which is provided with a plurality of light beams and generates an output signal in the relationship of Table 2.
There will be described hereinafter a system construction in which a solenoid valve for driving the slide of a press is controlled using the aforementioned fail-safe sensor.
The principle of fail-safe machine operation control described hereinafter is disclosed in U.S. Pat. No. 5,345,138, where a sensor output signal, Se, is provided as a permission signal. On the other hand, proposed such as by U.S. Pat. No. 5,285,721 is a fail-safe operation control circuit for a press.
FIGS. 10 and 11 show a constitutional principle in the case in which fail-safe operation control of a press is performed by means of an output signal, Se, of a sensor.
In the circuit configuration of FIG. 10, the output SI (electric current) of solenoid driving circuit is generated only when an operation instruction M exists and the output signal, Se, of the sensor (optical curtain) is being generated. FIG. 11 shows a time chart of the respective signals in the circuit of FIG. 10. When the presence and absence of generation of the respective signals M, Se, Si, SI are represented by logical values "1" and value "0", respectively, the logical relationship between the respective output signals is provided in Table 4.
TABLE 4 ______________________________________ M Se Si SI ______________________________________ 1 1 1 1 1 0 0 0 0 1 0 0 0 0 0 0 ______________________________________
As understood from Table 4, the output signal SI becomes a logical value "1" only when both of the input signals M, Se of the AND gate have logical values of "1", while the output signal SI assuredly becomes logical value "0" if either of the input signal M or Se is logical value "0", which means that a NOT operation is not included. Namely, in the constitution of FIG. 10, the output and input sides are connected with each other through the aforementioned monotonous logical relationship.
A normally closed solenoid valve is used within the press, the valve being one in which the movable iron core (plunger) is lifted up to open the valve thereby passing a pressure when its coil L is supplied with an electric current; the iron core will drop by gravity to close the valve when the electric current is cut off. In the system construction of FIG. 10, there is no function for stopping output based on the sensor output. Such a system is called a "normally closed system". The best known example of a "normally closed system" is an electromagnetic clutch brake.
Shown in FIGS. 12 and 13 are constitutional examples of machine operation control circuits which have a function to stop output based on the sensor output.
FIG. 12 shows a concrete example of a circuit in which a contact point "r" of electromagnetic relay Re for shutting off the electric current is inserted in series in an electric current supply circuit of a coil L for the solenoid valve.
In the constitution of FIG. 12, a NOT circuit N' is to be necessarily inserted, so as to shut off the electric current of the coil L when a danger is indicated, i.e., when the output signal SI should have a logical value "0". Namely, by the provision of NOT circuit N', the electromagnetic relay Re is supplied with an electric current thereby opening (turning OFF) the contact point "r" to thereby shut off the output signal SI, when the sensor output signal Se has disappeared to become logical value "0" (indicative of danger). When the sensor output signal Se has the logical value "1" (indicative of safe status) the electromagnetic relay Re is not supplied with electric current, so that the contact point "r" is kept closed (i.e., ON). Thus, the NOT circuit N', electromagnetic relay Re, and contact point "r" thereof cooperatively constitute an output stoppage circuit for stopping the actuation of a solenoid valve when the sensor output signal Se has indicated a danger (logical value "0"). This output stoppage circuit is constituted such that the electromagnetic relay Re is not supplied with electric current when safe status is indicated, i.e., under the normal condition, whereas an electric current is supplied thereto only when a dangerous situation has been detected. This type of system construction adopting the output stoppage circuit of FIG. 12 is called a "normally open system". This normally open system includes the NOT circuit N'. Thus, for example, even if the output signal SI should be shut off when the sensor output signal Se becomes a logical value "0", the contact point "r" may be kept closed (kept ON), since the electromagnetic relay Re is not excited, if a disconnection fault has occurred in the input or output line of the NOT circuit N'. Therefore, those system constitutions having such an output stoppage circuit configuration explained above can not be used in a fail-safe system.
FIG. 13 shows the circuit configuration of FIG. 12, as a logical circuit. Further, Table 5 shows a truth value for the respective signals in FIG. 13.
TABLE 5 ______________________________________ M Se /Se SI ______________________________________ 1 1 0 1 0 1 0 0 1 0 1 0 0 0 1 0 ______________________________________
In Table 5, the signal /Se is one which is renewedly formed or generated by the output stoppage circuit which is constituted of the NOT circuit N', electromagnetic relay Re, and contact point "r". In the logical circuit of FIG. 13, the logical relationship is nominally identical with that of Table 4, which relationship is between the operation instruction M, the sensor output signal Se, and the output signal SI of the solenoid driving circuit, but this logical circuit internally has a dual-step NOT operation function. Thus, a monotonous relationship is not obtained (i.e., there is included a process for converting the logical value "1", into "0", and the logical value "0" into "1") due to the interposition of the signal /Se by the output stoppage function, as understood from the truth value table of Table 5 which was prepared by including the internal constitution. As such, the light-beam type safety apparatus described in Japanese Examined Patent Publication No. 7-92193 has not been constituted in a fail-safe manner, since its system constitution has such a NOT operation function as explained above.
Namely, the light-beam type safety apparatus described in Japanese Examined Patent Publication No. 7-92193 is provided with a stop signal generation circuit, a foreign matter escape signal generation circuit, and a stoppage releasing signal generation circuit. The most important feature of this apparatus is that it has the stoppage releasing signal generation circuit for starting the operation of the press, and the stop signal generation circuit for stopping the operation of the press, and that the starting and stopping are instructed using different routes in the operation of the press. This system is also essentially unable to be regarded as being fail-safe, in that it has an output stoppage circuit such as that shown in FIGS. 12 and 13. In fact, in the circuit configuration shown in FIG. 1 of the Japanese Examined Patent Publication No. 7-92193, the interception signal for the light to be input to the stop signal generation circuit is supplied by the negation of the light receiving signal. As such, the stop signal is not generated if a disconnection fault of the input or output line of the stop signal generation circuit has occurred. Thus, the press can not be stopped, no matter how the aforementioned three circuit elements are constituted. Besides, its constitution is more complicated when compared with the basic constitution of a fail-safe system of FIGS. 10 and 11. When adopting any conventional light-beam type safety apparatus as a fail-safe operation control circuit, such a circuit should be constituted such that the stoppage releasing signal is not generated when the output stoppage signal can not be generated. Thus, those conventional light-beam type safety apparatuses are insufficient as a safety apparatus for a press, and are considered to be outside of the basic principle for securing safety.